SkDocument: use `auto` for MakeDocument().

A later CL will make this return a unique_ptr<SkDocument>.

Bug: skia:5972
Change-Id: Ie10d6c07d5f2524ecb71d906db0d37427827225d
Reviewed-on: https://skia-review.googlesource.com/c/181660
Reviewed-by: Hal Canary <halcanary@google.com>
Commit-Queue: Hal Canary <halcanary@google.com>
diff --git a/tests/PDFMetadataAttributeTest.cpp b/tests/PDFMetadataAttributeTest.cpp
index 1a51217..964d0e7 100644
--- a/tests/PDFMetadataAttributeTest.cpp
+++ b/tests/PDFMetadataAttributeTest.cpp
@@ -23,7 +23,7 @@
     metadata.fModified = now;
 
     SkDynamicMemoryWStream pdf;
-    sk_sp<SkDocument> doc = SkPDF::MakeDocument(&pdf, metadata);
+    auto doc = SkPDF::MakeDocument(&pdf, metadata);
     doc->beginPage(612.0f, 792.0f);
     doc->close();
     sk_sp<SkData> data = pdf.detachAsData();